An image sensor consists of an array of light sensitive picture elements (pixels) arranged in a sensor array region and periphery circuit elements. The pixels of the sensor array region respond to light incident on the pixels from a subject scene by generating electrical charges. The periphery circuit elements receive and process the generated electrical charges to display an image of the subject scene.
Image sensors may be fabricated on a semiconductor substrate using complementary metal-oxide-semiconductor (CMOS) circuits and fabrication techniques. In CMOS image sensors, each pixel consists of a photodiode formed on a semiconductor substrate and additional layers formed on the photodiode. These additional layers include one or more dielectric layers and metal layers to provide interconnects between the pixels and the periphery circuit elements. The side of the image sensor on which these additional layers are formed is referred to as the front side, while the side having the semiconductor substrate is referred to as the backside. In front-side illuminated (FSI) image sensors, light from the subject scene is incident on the front-side of the image sensors. However, the presence of the dielectric and metal layers on the front-side may limit the amount of light absorbed by the photodiodes, resulting in decreased sensitivity and degraded performance. In backside illuminated (BSI) image sensors, light is incident on the backside to allow a more direct path for the photons to reach the photodiodes. Thus, BSI CMOS image sensors avoid the obstruction to the optical path by the front-side layers so as to increase the number of photons reaching the photodiodes.
To improve light sensitivity of BSI CMOS image sensors, the semiconductor substrate is typically thinned. Furthermore, a thin layer of P+ ions may be implanted on the backside of the thinned semiconductor substrate to increase the number of photons converted into electrical charges. Once the thin P+ layer 122 is formed, a laser annealing step is performed to repair crystal defects caused by the ion implantation step and to activate the implanted P+ ions. Laser annealing is typically performed by scanning a laser beam in a scan pattern on a wafer containing an array of BSI image sensors. Uniformity of laser annealing is dependent on the uniformity of energy projected on the wafer from the scanning laser beam. However, laser beams typically do not have a uniform distribution of energy across the beam width. For example, energy density of a laser beam is usually reduced near the beam boundary. As a result, a sensor array region may not be uniformly annealed if it overlaps with a boundary of the laser scan pattern. This boundary effect may introduce dark current, which is current generated in the sensor array region even in the complete absence of incident light. Dark current causes horizontal and/or vertical stripe patterns in the image. It also adversely affects image sensor performance by making it more difficult for the sensor array region to detect light. Accordingly, there is a need to control the laser beam size and the scan pattern of laser annealing to keep the boundary effect from occurring within the sensor array region of an image sensor.